Communications switching systems—such as private branch exchanges (PBXs), for instance—provide a variety of call-hold, transfer, and conferencing features to their users. Some also offer a feature where a user can call the PBX from an external (e.g., public network) phone and then use the PBX facilities to make internal or external calls, usually in order to reduce toll charges.
When users' telephones are wireline phones, there is no danger, and hence no expectation, that calls will be dropped due to lost signal strength, although calls may be dropped for other reasons. That is not the case with wireless phones, however. In the case of wireless phones, a user's access to the various system features is dependent on the radio signal remaining strong enough to maintain the call connection. When signal strength deteriorates below some threshold, the connection is dropped at one end and the call is lost. In some systems and under certain circumstances, a wireless phone may attempt to reestablish a dropped connection. However, the connection is usually reestablished as a new call and not as a continuation of the prior call. Likewise, a wireline call may establish a new call in place of a dropped call.
The Digital HotDesk by Avaya Inc. is a private branch exchange (PBX) application that extends PBX-like functionality to off-net telephony endpoints such as cellular phones. It has a cellular cut-off feature wherein, when the caller has the feature enabled in his or her profile and the call terminates at a called mobile location, Digital HotDesk monitors the state of a call, and will try to ascertain whether a Cellular Cut-Off of the called party has occurred during a conversation. It does this by waiting a specified period of time after the called party has cleared down, when the caller is still holding on the call. If the time period is exceeded, the fact that the call was cut off is logged in the call billing record and HotDesk automatically places a call back to the called party at the number that the called party disconnected from, and plays a voice message to the caller to inform him or her that the HotDesk is trying to reconnect the call. The system will try the called party for a given time period, before placing the caller either in caller options or in the called party's mailbox. When a call that has been cut-off is presented back to the called user, it is announced like any other HotDesk call. If a reconnection is established, it is given a new call billing record but the same call handle as the cut-off call so that the two calls can be linked. For HotDesk users participating in conference calls on their mobile phones with Cellular Cut-Off enabled, if the user wants to leave the call prior to the end of the conference, the system will see the user hanging-up as a Cellular Cut-Off, and try and re-connect the call while playing a prompt to the remaining conference participants. The call will then either be answered by the user or by voice mail. It is therefore recommended that users likely to participate in conference calls from their mobile phone have the Cellular Cut-Off feature disabled.
While going a long way to solve the problem of dropped calls, the Cellular Cut-Off feature is limited in its capabilities, restoring calls only to called parties at the called numbers. For example, it works only for called parties and not for calling parties; it cannot change the phone number or the medium of the connection that is reestablished to the called party; and it does not preserve the context of the call.